A known filter for collecting the particulates discharged by a diesel engine, or the like is disclosed in, for example, Japanese Patent Application Laid-Open under No. 131518/1980 or 170516/1983.
This filter is made of a ceramic material, and is provided upstream thereof with a regenerating heater for removing particulates by burning from the filter and thereby regenerating it.
There has also been proposed a filter provided with a regenerating burner, instead of the heater, for blowing a flame against the filter to regenerate it.
There has also been proposed a self-heating filter comprising a ceramic honeycomb which is formed from electrically conductive silicon carbide and can heat itself electrically for regeneration by burning particulates (Japanese Patent Application Laid-Open under No. 110311/1982).
The filter mentioned above as a first example of the known devices has, however, been found to present a number of problems. It is necessary to detect accurately the amount of particulates collected by the filter. If the amount is too small, fire goes out sooner than the filter can be regenerated, but if it is too large, the filter cracks or melts down. The filter is easily broken, since it is likely to have a temperature distribution lacking uniformity, and is also low in mechanical strength.
The filter mentioned as a second example requires the burner as special equipment. Moreover, it presents the same problems of cracking, etc. as the first example of filter does.
The self-heating filter mentioned as a third example has the advantage that it can be regenerated without the aid of any special equipment, insofar as it can heat itself.
This filter is, however, difficult to manufacture, since silicon carbide shows a high degree of shrinkage when a molded product thereof is fired. Moreover, it is easily broken during its regeneration under heat. For these and other reasons, the filter has been available only in a limited variety of shapes, or with a great deal of difficulty in the selection of an appropriate value of electrical resistance.
Under these circumstances, it is an object of this invention to provide a self-heating filter which has a high degree of strength to withstand regeneration under heat and a high efficiency in the collection of fine particles, and is easy to manufacture.